[libcxx] [libcxxabi] [libunwind] [runtimes] Always define cxx_shared, cxx_static & other targets (PR #80007)

[Mark Murray via cfe-commits]

MarkMurrayARM wrote:

Hi - is this being worked on? We are still seeing build failures that look like
```
-- ABI list file not generated for configuration aarch64-none-elf.libcxxabi.v2.unstable.exceptions.nonew, `check-cxx-abilist` will not be available.
-- Configuring done
-- Generating done
CMake Error:
  Running

   '/usr/bin/ninja' '-C' '/home/marmur02/oss/LLVM-embedded-toolchain-for-Arm/build/libcxx_libcxxabi_libunwind/aarch64a_exn_rtti/src/libcxx_libcxxabi_libunwind_aarch64a_exn_rtti-build' '-t' 'recompact'

  failed with:
   ninja: error: build.ninja:524: multiple rules generate lib/libunwind.a [-w dupbuild=err]
```
... which don't happen with this change locally backed out.

https://github.com/llvm/llvm-project/pull/80007